Method, Apparatus and System for Sensing a Signal with Automatic Adjustments for Changing Signal Levels

ABSTRACT

The present specification provides a method, apparatus and system for sensing a signal with automatic adjustments for changing signal levels. A novel fractional peak discriminator circuit is provided which can be incorporated into a system for measuring periodic signals from moving elements. The circuit can be used regardless of whether the periodic signals are detected using optics, magnetic detector or other methods.

PRIORITY

This application is a continuation of U.S. patent application Ser. No.13/510,202, filed Nov. 21, 2012, which claims priority from U.S.Provisional Patent Application 61/262,365 filed Nov. 18, 2009, theentire contents of which are incorporated herein by reference.

FIELD

The present specification relates generally to signal processing andmore specifically relates to a method, apparatus and system for sensinga signal with automatic adjustments for changing signal levels.

BACKGROUND

Various methods have been used to track the rotational speed of anythingfrom car wheels to turbine engines. There are various approaches forcapturing these rotational speeds. One approach is to attach a smallgenerator, to a shaft or other rotating device which puts out a voltageproportional to the speed the generator is turning. This approach isoften not desirable as it involves an additional mechanical connection.

Another approach is a magnetic tachometer having a Hall-Effect sensor,which changes voltage when a magnetic field passing through the sensorchanges. The voltage output is used to trigger an electronic circuit.These devices depend on the pulse being of a certain size to trigger thecircuit and any attendant noise on the signal wire to be small enoughnot to trigger the circuit.

Another approach is optical. In this case a photo transistor or photodiode senses reflected light and when the light increases or decreases,a change in current occurs in the attached circuit. This current istranslated into a voltage, which is captured as noted above.

In these approaches there are three common factors: a. A rotating oroscillating machine; b. Coupling method (mechanical, magnetic, optical);c. A Circuit for detecting a voltage, voltage change, or current.

The electronics related to these approaches are tasked with ignoringelectronic noise and detecting a true signal. Various methods have beenused to achieve these tasks, but common methods comprise filtering noiseelectronically, while ensuring the signal level would be great enough orof sufficiently different frequency not to be filtered or ignored.

Of these approaches, optical systems are often selected. Optical systemsinclude a light source and a photo sensor and a detector. The photosensor can be, for example, a phototransistor or a photodiode or acharge coupled device (CCD). Light is emitted from the light source ontothe moving part. (“Moving” captures all types of movement, includingrotations and oscillations). The photo sensor captures reflections. Thedetector detects a difference contrast (lightness or darkness) on themoving.

In general, optical approaches face the problem of having enough lightilluminating something of sufficient contrast to provide a signal bigenough above the ‘noise floor’ to be considered valid. Morespecifically, problems involved in the optical approach includesufficient light; a paint spot, marked tape, color patch, (or some otheroptically differentiating part of the surface that add a differentreflectance to the illuminating light source, and as detected by thephoto sensor) which didn't fall off, fade, become tarnished, dirty, ordiscolored; and an electronic circuit that was tolerant of possiblechanges over time, optical path changes/variations, and rotating speeds.General electronics filtering and technology can be used for thesepurposes.

One method of handling these signals, since they do not change markedly,is to run them through a Phase-Lock-Loop. This is an electronic circuitthat ‘seeks’ to oscillate in phase and at the same frequency as anincoming signal; but if there are some skips or small variations in theincoming signals, it will keep the output frequency steady. Thus, it isnoise tolerant. This can be used since the signal does not changefrequency suddenly. A car, for instance, will not come to a halt withoutbraking, nor instantly go to sixty miles per hour without acceleratingto that speed. (The only times things stop suddenly is because they hitsomething and the output of the tachometer is not likely to be instanceunder such circumstances.)

Another signal processing method is to take the AC signal (i.e., thechanging part of the signal as opposed to the average or DC signal) andcompare its crests to another voltage and when the crest exceeded thecomparing voltage, an output pulse would be generated by the electronicsto the system monitoring the speed. However this works when the signaldoes not change amplitude appreciably. As the moving mechanical partincreases in speed, there is a tendency for the signal to get small,since the reflected light or magnetic field is passing more quickly. Bythe same token, as the moving mechanical part decreases in speed, thereis a tendency for the signal to get large. However, this requires thatan operator turn a knob (variable resistor) to adjust the comparisonvoltage to the right level to get valid output pulses.

SUMMARY

The present specification provides a signal conditioner circuitcomprising:

a filter for receiving an electrical signal generated by a sensor; saidsensor configured to detect periodic movement and generate saidelectrical signal based on said periodic movement; said filter forgenerating a filtered signal having a peak where said electrical signalis greatest; a peak detector configured to receive said filtered signaland to detect said peak and generate a detected-peak signal that holdssaid peak; a peak-divider configured to receive said detected-peaksignal and to divide said detected-peak signal by a predetermined amountand thereby generate a divided-peak signal; a comparator configured toreceive said divided-peak signal and said filtered signal; saidcomparator configured to generate and output a pulse when a comparisonbetween said divided-peak signal and said filtered signal results in adetermination that said filtered signal exceeds said divided-peaksignal.

The circuit can further comprise a pulse generator configured togenerate a further pulse, based on said pulse; said further pulseconditioned for monitoring equipment.

The sensor can be an optical sensor.

The sensor can be a magnetic sensor.

The periodic movement can be rotational. Each peak can represent asingle rotation, or a plurality of said peaks can represent a singlerotation. The periodic movement may correspond to rotation of a turbinein a jet engine.

The periodic movement can be oscillatory. The periodic movement cancorrespond to reciprocating movement of a piston.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a signal sensing system in accordance with a firstembodiment.

FIG. 2 shows a signal sensing system in accordance with anotherembodiment.

FIG. 3 shows a block diagram of an exemplary implementation of thesignal conditioner circuit of FIG. 1 and FIG. 2.

FIG. 4 shows a specific circuit diagram giving an example of how theblock diagram of FIG. 3 can be implemented.

FIG. 5 shows another specific circuit diagram giving an example of howthe block diagram of FIG. 3 can be implemented.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Referring to FIG. 1, a signal sensing system is indicated generally at50. System 50 comprises a light source 54, a light sensor 58 and asignal conditioner circuit 62 connected to the light sensor via a link66. A moving element 70 is provided having an optical marker 74 disposedthereon. In a present example, moving element 70 is rotating in thedirection “A”.

Light source 54 emits light 78 which is reflected off the surface ofelement 70. Different amounts of light are reflected each time marker 74passes in front of light 78. Light source can be from a fibre opticcable, or ambient light, or other source. Light sensor 58 capturesreflected light 79 as it is reflected from element 70. Link 66 may be asignal wire or even a radio channel. In general link 66 is configured tointroduce as little error as possible to any signal 80 captured by lightsensor 58.

System 50 is a simplified illustrative example. It is to be understoodhowever that the term “move” and its variants (e.g. “moving”) can referto any type of movement, including rotation and oscillation. A piston isan example of an oscillating element. In a practical application, movingelement 70 can be, for example, an engine turbine used on an aircraft.On review of this specification, other practical applications will occurto those skilled in the art.

Notable characteristics of a periodic signal from a rotating oroscillating object, such as element 70, are that a periodic signal doeschange markedly in amplitude or frequency and that most noise is at mostabout sixty percent of the real signal amplitude. (If suchcharacteristics are not met, the desired functionality from the rotatingelement will be nearly unworkable in any case.) Based on thischaracteristic, circuit 62 can be configured to capture the present peaksignal, and then a fixed fraction of that signal can be used to validatea rotational signal. Such a circuit 62 can be configured to adapt to thetime, speed, and optical/magnetic variations that can occur intachometer systems.

Optical marker 74 is any type of contrasting mark such as a reflectivetape, or paint, which changes the level of reflected light 79.

Light sensor 58 can be implemented as a phototransistor, photodiode, orcharge couple device (CCD) or the like. Light sensor 58 capturesreflected light 79 and generates an electrical signal 80 (e.g. voltageor current) that is substantially proportional to the amount ofreflected light 79 captured by light sensor 58.

Signal conditioner circuit 62 receives the electrical signal from sensor58. Signal conditioner circuit 62, which may be referred to as afractional peak discriminator circuit, processes the electrical signalfrom sensor 58 and outputs an output signal to monitoring equipment (notshown).

Signal conditioner circuit 62 will be discussed in greater detail below.However, before proceeding further it is to be understood that othertypes of sensing modalities may be used to obtain the electrical signalthat is processed by signal conditioner circuit 62. Referring now toFIG. 2, another signal sensing system is indicated generally at 50 a.System 50 a is a variant on system 50, and so like elements bear likereferences except followed by the suffix “a”.

Of note is that in system 50 a, light source 54 and marker 74 areeliminated. In their place, a magnetic element 75 a is provided on thesurface of moving element 70 a. Magnetic element 75 a emits a magneticfield 81 a, which is periodically detected by a magnetic sensor 58 a,used in place of optical sensor 58. Magnetic sensor 58 a is thusconfigured to generate an electrical signal 80 a. Magnetic sensor 58 acan be based on a hall-effect detector, in which case a voltage signalis generated that is proportional to the detected magnetic field 81 a.Magnetic sensor 58 a thus generates an electrical signal 80 a that isreceived by signal conditioner circuit 62 a.

FIG. 3 shows a block diagram representing a possible implementation forsignal conditioner circuit 62 (or signal conditioner circuit 62 a).Signal conditioner circuit 62 comprises a filter 100 which receivessignal 80 a via link 66. Filter 100 input sends filtered signal 102 to apeak detector 104 and a comparator 108. The detected-peak signal 106from peak detector 104 provides input to peak divider 112. Thedivided-peak signal 114 provides a second input to comparator 108, whichis configured to make a comparison between filtered signal 102 anddivided-peak signal 114. As will be discussed further below, comparator108 will generate a pulse when a comparison 109 results in adetermination that filtered signal 102 exceeds divided-peak signal 114.The compared-signal 110 outputted from comparator 108 provides input topulse generator 116. Generated-pulse signals 118 are outputted frompulse generator 116 and provide input to monitoring equipment (notshown).

The operation of signal conditioner circuit 62 will now be discussed ingreater detail, which will also provide further understanding as to howsignal conditioner circuit 62 may be constructed. As noted above, signal80 comes from a photo-sensor 58 or magnetic sensor 58 a or other type ofvoltage or current-generating device.

It is contemplated that system 50 may be located within a noisyenvironment and so noise may be introduced on link 66 or elsewhere,resulting in the acquisition of noise on link 66 (or elsewhere) whichwill be outside the frequency of interest for the purposes of thetachometer. Accordingly, signal 80 is filtered at filter 100 as aprecautionary design practice to reduce or eliminate frequencies outsidethose of interest. “AC bypassing” techniques and “RC Filtering”techniques can be usual for these purposes. To add dynamic range, it canbe desired to amplify the filtered version of signal 100 before doingany peak detection.

Filter 100 is thus configured to generate filtered signal 102, which is“well behaved”, in that it signal 102 shows a crest or peak where theoptical or magnetic return is greatest. N system 50 or system 50 a, thiscrest or peak may be a once per revolution. However, multiple peaks mayoccur where a plurality of markers 74 (or magnetic elements 75 a) areemployed. It will now be apparent that the number of markers 74 (ormagnetic elements 75 a) can be selected according to the differentdesign specifications for system 50 or system 50 a.

Peak detector 104 comprises a peak-and-hold circuit, which can beimplemented through the use of an operational amplifier to impress avoltage on a capacitor as the filtered signal 102 rises to a peak. Adiode can also be provided to prevent (or at least reduce the likelihoodof) the capacitor from discharging as the voltage declines from thepeak, thus storing a voltage charge on the capacitor substantially equalto the peak of the input and filtered signal. This voltage level istransmitted to a voltage buffering circuit (also known as a voltagefollower) which isolates the capacitor from discharging.

Peak divider 112 then buffers and divides the detected peak signal 106(i.e. the peak voltage) using a voltage divider circuit, which can beimplemented using two resistors in series. This can be an adjustablepoint on the circuit so that any percentage of the peak can be utilizedas a comparison voltage to the signal peaks that follow.

Divided-peak signal 114 is the transmitted to comparator 108 to becompared with filtered signal 102. Comparator 108 is thus provided withthe incoming signal train of pulses representing the optical pulses (ormagnetic pulses) of element 70, and a percentage of the peak of theprevious signal. Since the signal peaks are fairly constant fromcycle-to-cycle, this is a substantially reliable method of detecting thenext peak of the signal.

Comparator 108 thus determines when signal 102 is greater than thedivided-peak signal 114. Compared-signal 110 will thus be a ‘high’voltage (such as 2.4 volts to 5 volts) during the period when theincoming signal is of a greater voltage than the chosen percentage ofthe peak signal. Compared-signal 110 is transmitted to pulse generator116.

Compared-signal 110 may or may not be sufficient to meet therequirements of the equipment monitoring the rotation speed of themechanical system, therefore a pulse generator 116 is provided whichmeets the voltage and amplitude needs of the monitoring equipment. Itwill now be apparent though that depending on the monitoring equipment,pulse generator 116 may be obviated.

Generated pulse signals 118 are then sent to the monitoring equipment,which lets the monitoring equipment know when the optical marker 74 (ormagnetic element 75 a) has been detected moving past on the movingelement 70.

Referring now to FIG. 4 and Table I, a specific but non-limiting exampleof how circuit 62 can be implemented is provided, which is indicatedgenerally as circuit 62 b.

TABLE I Block Element Part Reference Part Description Filter 100 R1Resistor, 10 kohms, 5%, 0.1 Watt U1B Filter 100 R2 Resistor, 100 ohms,5%, 0.1 Watt Filter 100 C1 Capacitor 0.1 uF, 25 V, 10% Filter 100 O1Integrated Circuit Operational Amplifier; Prec JFET Peak-detector 104 D1Schottky Diode, 100 mA, VR = 45 V Peak-detector 104 C4 Capacitor 1 uF,16 V, 10%, C1206 Peak-detector 104 O2 Integrated Circuit OperationalAmplifier Prec JFET Peak Divider 112 R10 Resistor, 5 Kohms Peak Divider112 O3 Integrated Circuit Operational Amplifier Prec JFET Comparator 108R11 Resistor 15 Kohms, 5%, 0.1 W Comparator 108 C9 Capacitor 0.01 uFComparator 108 O4 Operational Amplifier LT3941S8

Referring now to FIG. 5 and Table II, a further specific, butnon-limiting example of how circuit 62 c can be implemented is provided.

TABLE II Block Element Part Reference Part Description Filter 100 R1Resistor, 10 kohms, 5%, 0.1 Watt U1B Filter 100 R2 Resistor, 100 ohms,5%, 0.1 Watt Filter 100 C1 Capacitor 0.1 uF, 25 V, 10% Filter 100 O1Integrated Circuit Operational Amplifier; Prec JFET Peak-detector 104 D1Schottky Diode, 100 mA, VR = 45 V Peak-detector 104 C4 Capacitor 1 uF,16 V, 10%, C1206 Peak-detector 104 O2 Integrated Circuit OperationalAmplifier Prec JFET Peak Divider 112 R10 Resistor, 5 Kohms Peak Divider112 O3 Integrated Circuit Operational Amplifier Prec JFET Peak Divider112 C51 Capacitor, 1.0 uF, 25 V, 5% Peak Divider 112 R32 Resistor, 25.5Kohms, 1%, 0.1 W Peak Divider 112 R33 Resistor, 25.5 Kohms, 1%, 0.1 WPeak Divider 112 O5 Integrated Circuit Operational Amplifier Prec JFETComparator 108 R11 Resistor 15 Kohms, 5%, 0.1 W Comparator 108 C9Capacitor 0.01 uF Comparator 108 O4 Operational Amplifier LT3941S8

In circuit 62 c of FIG. 5, modifications were made to the peak divider112 to reduce (and, as much as possible, minimize) jitter in thedivided-peak signal 114.

This was achieved by operational amplifier 05 and the associatedcomponents as shown in FIG. 5. The added amplifier's output is summedwith (connected to) the output of the voltage buffering amplifier tocreate a Buffered Peak Signal.

It will now be apparent that one of the advantages of provided by thisspecification is a means to sense of an optical or magnetic signal thatautomatically adjusts for a changing signal level, but not incorrectlytrigger on random electronic noise.

Combinations, subsets and variations of the foregoing are contemplated.

1. A signal conditioner circuit comprising: a filter for receiving anelectrical signal generated by a sensor; said sensor configured todetect periodic oscillatory movement and generate said electrical signalbased on said periodic oscillatory movement; said filter for generatinga filtered signal having a peak where said electrical signal isgreatest; a peak detector configured to receive said filtered signal andto detect said peak and generate a detected-peak signal that holds saidpeak; a peak-divider configured to receive said detected-peak signal andto divide said detected-peak signal by a predetermined amount andthereby generate a divided-peak signal; a comparator configured toreceive said divided-peak signal and said filtered signal; saidcomparator configured to generate and output a pulse when a comparisonbetween said divided-peak signal and said filtered signal results in adetermination that said filtered signal exceeds said divided-peaksignal.
 2. The circuit of claim 1 further comprising a pulse generatorconfigured to generate a further pulse, based on said pulse; saidfurther pulse conditioned for monitoring equipment.
 3. The circuit ofclaim 1 wherein said sensor is an optical sensor.
 4. The circuit ofclaim 1 wherein said sensor is a magnetic sensor. 5-9. (canceled) 10.The circuit of claim 1 wherein said periodic oscillatory movementcorresponds to reciprocating movement of a piston.
 11. A signalconditioner circuit comprising: a filter for receiving an electricalsignal generated by an optical sensor; said optical sensor configured todetect periodic movement and generate said electrical signal based onsaid periodic movement; said filter for generating a filtered signalhaving a peak where said electrical signal is greatest; a peak detectorconfigured to receive said filtered signal and to detect said peak andgenerate a detected-peak signal that holds said peak; a peak-dividerconfigured to receive said detected-peak signal and to divide saiddetected-peak signal by a predetermined amount and thereby generate adivided-peak signal; a comparator configured to receive saiddivided-peak signal and said filtered signal; said comparator configuredto generate and output a pulse when a comparison between saiddivided-peak signal and said filtered signal results in a determinationthat said filtered signal exceeds said divided-peak signal.
 12. Thecircuit of claim 11 further comprising a pulse generator configured togenerate a further pulse, based on said pulse; said further pulseconditioned for monitoring equipment.
 13. The circuit of claim 11wherein said periodic movement is rotational.
 14. The circuit of claim13 wherein each said peak represents a single rotation.
 15. The circuitof claim 13 wherein a plurality of said peaks represents a singlerotation.
 16. The circuit of claim 13 wherein said periodic movementcorresponds to rotation of a turbine in a jet engine.
 17. The circuit ofclaim 11 wherein said periodic movement corresponds to reciprocatingmovement of a piston.